CODEWITHSUNDEEP
network-programmingtcpip
abrumda
abrumda

Reputation: 1

Why is it not possible to spoof an ip address (without using a proxy) and still receive a response?

I understand that if I tell my computer to send TCP packets from a fake ip address - say 128.5.32.3 - then my computer will happily send the packets out but not receive them in response.

But why is no response received? At which point in the chain is the return packet dropped?

Or, to give the same question asked another way - if my internet provider assigns me some arbitrary IP address, why can't my computer tell the internet provider to give me a different, arbitrary, IP address?

Upvotes: 0

Views: 1456

Answers (3)

The Dude
The Dude

Reputation: 1046

There are many reasons why an ISP will not give an 'arbitrary' IP address. These include

  1. They themselves only have a block of IP addresses they are allowed to allocate to users, if the IP address you want to use is not in this block there's nothing they can do (even if they want to, which they probably don't)
  2. You are mostly likely being assigned an IP through DHCP (unless your provider is very generous or you are paying for a static IP). This also means that your IP is frequently changing.

The reason you receive no response is, as you put it, because the spoofed address is not your IP address. You are in essence telling the receiver of the TCP packets to respond to a different user (e.g., you send a packet, and they respond to some random stranger).

Upvotes: 0

VortixDev
VortixDev

Reputation: 1013

To start with your question about why no response is received, it is because the response goes to the person whose IP you spoofed. This can be abused, and an example if this is a "smurf attack". You would need to control the spoofed IP in order to receive the response, and there would be no point to spoofing if you had this control.

As for your question about why you cannot make your ISP assign you an IP is because, firstly, your ISP has control of a range of IPs and cannot assign IPs out of its permitted range. Secondly, most ISPs won't take into account the IP that your device wants. It has full control and will control your IP how it wants, so you cannot change your external IP at will.

Upvotes: 0

Joseph Hopkins
Joseph Hopkins

Reputation: 117

It's like sending a letter with a return address in it that is invalid. The mail will still get there, but if they send it back the postman (router) will at best be able to deliver it to a fake return address.

Your internet provider gives you an address on internet that isn't arbitrary rather one of it's internet addresses it has allocated. You can't 'move house' by wishing it.

If you do move house by getting another valid address you still need to receive a response using address supplied.

The postmen (routers) are incorruptible AFAIK :)

Upvotes: 2

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